Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
1998-12-08
2001-08-21
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C435S430100
Reexamination Certificate
active
06278314
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electric circuit comprising a plurality of high power switching devices connected in series and means adapted to control them to be all conducting or blocking substantially simultaneously to act together as one single switch and to each hold a part of a high voltage to be held by the switch in the blocking state of the switching devices. Each switching device has a rectifying element connected in anti-parallel therewith.
BACKGROUND OF THE INVENTION
Such electric circuits are for example used in different types of converters, rectifiers and inverters, in which high voltages are to be handled. The use of such an electric circuit in a converter in a station for converting direct voltage into alternating voltage may be mentioned as a non-limitative example.
The property in common to all these applications is that the voltages to be held in the blocking state of the single switch are too high to be held by one single switching device, so that a plurality of such devices has to be connected in series for distributing the voltage to be held by the switch thereamong. The voltage to be held may for instance be 30-500 kV, and each switching device may for example alone only hold 1-10 kV. The problem is that it is not possible to manufacture switching devices so that they have exactly the same properties, i.e. reacts exactly the same in every single instant. This means that an over-voltage, i.e. a voltage being higher than the average voltage to be held by the switching devices, may easily occur for an individual switching device during switching due to differences in turn-on delay times, turn-off delay times, tail currents and reverse recovery charges of opposite diodes in a phase leg of a voltage source converter in the case that the single switch forms a current valve of such a converter. It is essential to protect the different switching devices against such overvoltages, since they may destroy the device and possibly result in a drop out of a larger unit, such as a converter station with an interruption of the operation thereof being very costly as a consequence.
It is not a realistic solution to this problem to arrange that many switching devices in series in such a switch without additional surrounding components that the over-voltages will always be within an acceptable limit, but it is a desire to keep the number of switching devices in such a single switch as low as possible for saving costs for the equipment for controlling the switching devices, cooling them and so on. It has therefore been necessary to design the means for controlling the switching devices so that over-voltages occurring will be kept within an acceptable area with respect to magnitude and duration, so that they will not be harmful to the switching devices. This normally means that each individual switching device has to be provided with a separate control member and the control of the single switch has been made very complex and intelligent gate drive circuitry limiting the voltage-time gradient has been utilized in order to control the quotient for each individual switching device. This means that the cost of those electric circuits already known is comparatively high. Another problem always existing in such electric circuits is that the power losses in different components are higher than desired.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an electric circuit of the type defined in the introduction, which solves the problems discussed above to a large extent.
The object according to the invention is achieved by providing such an electric circuit with an avalanche diode as the rectifying element adapted to start to conduct current when reverse biased as of a predetermined voltage limit for preventing that the voltage across the respective switching device from exceeding the limit. The avalanche diode is made of SiC. By producing such a diode of SiC, this may be given the properties required for being connected in anti-parallel with a high power switching device as an over-voltage protection. Accordingly, some superior properties of SiC with respect to, for example, Si are then used, since that SiC has a much higher breakdown voltage than Si and since an avalanche diode in SiC may therefore be made much thinner, which means considerably lower power losses. Such a diode of SiC may also withstand very high temperatures. This means that an avalanche diode may take a much higher current during a certain period of time than, for instance, Si when the voltage across the switching device, reaches a predetermined voltage limit. This way of obtaining an over-voltage suppression utilizing an avalanche diode connected in anti-parallel with the switching device and acting as a voltage clamp constitutes a very simple solution to the problems of over-voltages mentioned above. This means that the control of the switching devices is no longer that delicate and the means for this control may be simplified. Snubbers may also be very much simplified. This means, on one hand, that the costs of the entire electric circuit with control equipment will be considerably reduced, the reliability of the operation of the circuit will also be increased, and the risks of operation interruptions of a plant or the like including such a circuit will be reduced.
According to a preferred embodiment of the invention the avalanche diode has a predetermined voltage limit at a level being higher than a normal voltage to be held by the respective switching device at normal operation of the circuit and when the voltage to be held by a single switch is substantially equally distributed among the switching devices thereof, and a predetermined voltage is preferably at least 30% higher than the normal voltage. Accordingly, when the voltage over the single switch is equally distributed among the switching devices, no avalanche diode will be conducting, but the diodes are there for all the switching devices to prevent any of them from receiving a current exceeding the normal voltage by more than 30% at any time, that it is not so critical to control the different switching devices through complicated circuits for detecting different function parameters of the devices. It is advantageous that there is a considerable difference between the predetermined voltage limit and the normal voltage, so that the avalanche diode will not be conducting except when an abnormal condition prevails at a specific switching device, since the avalanche diode may otherwise be forced to conduct at too high a current during too long a time and, by that, be destroyed.
According to another preferred embodiment of the invention, the avalanche diode has a predetermined voltage limit at a level below a second voltage limit, exceeding which may be harmful to the respective switching device, and the level of a predetermined voltage limit is preferably at least 10% below the second voltage limit. This ensures that there will never be a voltage across any switching device to completely or partially destroy the device.
According to another preferred embodiment of the invention, the switching devices are IGBT's. IGBT's are preferred as switching devices in such an electric circuit, since they may very easily be turned on and off simultaneously, but the problems to be solved by the present invention are also there for such switching devices. According to another preferred embodiment of the invention the switching devices are MOSFET's. However, it is emphasized that the present invention covers electric circuits of the type defined in the introduction in which the switching devices may be of any type of power semiconductor component of turn-on and or turn-off type.
According to another preferred embodiment of the invention, the circuit is a converter circuit, the single switch forms a current valve thereof, and a free-wheeling diode is connected in anti-parallel with each switching device of the valve. This is a preferred application of the present invention.
According to another preferred emb
Åberg Anders
Asplund Gunnar
Danielsson Bo
Hermansson Willy
Lundberg Peter
ABB Research Ltd.
Connolly Bove Lodge & Hutz
Lam Tuan T.
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